NEW SPECIES OF EHRENBEKG S GEXUS AULISCUS. 5 



now prett}^ certain that Ehi'enberg's Aulisci have no apertures, and that, by correct- 

 ing his description as follows, it will include the beautiful forms represented in our 

 figures 3, 4, 5, 6, 7, 8, 9, and 13. The Eapodiscus scidplus, Smith, Briliih Dkdo- 

 maceae, p. 25, also belongs to this genus. 



AlJIilSCUS.— EURENBERG. 



Lorica cylindrical (often discoid) ; bases^ circular, undulated, having two circular, 

 flattened, mastoid, imperforate processes at some distance from the margin; umbi- 

 licus (generally present) smooth, circular, surrounded bj' a plumose arr<ingcment of 

 dots and lines ; sides smooth. The projections on one base are usually on a line at 

 right angles to that on which those of the opposite base are placed. 



The ibilowing species appear to belong here, and are, I believe, entirely new. 



1. Aulisciis priiinoi^iis, B. 



(Plate, Figs. 5, G, 7, and 8.) 



Lorica large; edges bevelled; bases marked with four sets of curved and sparsely 

 punctate lines, two sets of which diverge from the circumference of the large smooth 

 umbilicus, while the other two sets converge around each of the two large mastoid 

 processes. Sides smooth, or with distant lines purallel to the base. Diameter from 

 Yp\„th ( = 2"" ) to yji"^ o^th ( = ()" ) of an Englisli inch." This species occurs in 

 estuaries, &c. from Massachusetts to the Gulf of Mexico. I found it particularly 

 abundant at Ballast Point, Tampa Bay, Florida. It occurs in the Hudson 

 River at West Point, and in the earth of rice fields in Georgia and South Carolina. 



This species, when of largest diameter, is usuall}^ of a flattened or discoid Ibrm, 

 as in Figs. 5 and 6 ; but when smaller, it presents considerable variety in the lenuth 

 of its sides, sometimes being discoid, as in Fig. 6, and not unlVequently showing 

 long cylindrical sides marked by numerous distant lines, pai'allel to the base, as in 

 Fig. 7. Specimens frequently occur in which the sides show two or more cylin- 

 drical portions differing considerably in diameter, as in Fig. 7. These varieties all 

 occur together, and pass into each other by such gradations that I am satisfied they 

 .should all be referred to the same species. 



3. Anlisciis piinctatiis, 13. 



(Plate, Fig. 9.) 



Lorica, as in the preceding species, but having the lines so crowded and so closely 

 punctate that the plumose arrangement is scarcely .yisible. 



' la this paper, the tcnn base is applici] nearly in its geometrical sense to the circular or elliptical ends 

 of cylindrical forms; to the triangular, quadrangular, or curved cuds of Trk-eralium, Amphitelras, Z^/ijo- 

 cei-oK, &e. ; and, in the Naviculaceae and allied forms, the term base will be applied to the striated, grooved, 

 or punctate surfaces on.which the thickened portions usually, but erroneously called, apertures exist. That 

 the so-called apertures in Kavicida, Piimularia, Staui-oneis, &c. are, in reality, the thickest parts of the 

 shell, I proved, some time since, by the action of hydrofluoric acid in which these portions are the last 

 to dissolve. (See SilUman's Journal, 2d series, XI. 349.) 



" All dimensions in this paper will be given in thousandths of an English inch, so as to have the same 

 unit of comparison for all the figures. The small m attached to a number will bo used as a symbol for 

 TrVijth of an inch, thus: 6"" = jo'j^ths. 



